after chatting with grumpyvette, it got my gears turning. so i thought i'd add some more info.

one of the main reasons that you need a little more timing at idle and just off idle is that you are not filling the cylinders. when the engine is at low rpm, the throat of the carb is nearly closed, not letting much air into the cylinders. it's not filling the cylinders with the intake partially or almost fully restricted and thus when it's compressing the mix, there's less to compress, so there are much lower compression pressures. that lightly compressed mixture, inefficient to begin with as far as ideal A/F ratio, will burn more slowly than a highly compressed, mixed, swirled mixture at a more open throttle position. the charge physically can't mix as well as it does at higher rpms, as not only is there less air and fuel to mix, but the velocity of the incoming charge is lower. the lean mixture takes longer to burn, creating more heat in the cylinder, causing pre-ignition or pinging, which is your air/fuel charge being ignited by the hot spot in the cylinder, and not the spark plug, where and when it should be.

for instance, if you light a fire from many places, it will burn faster than if you light it at one. this may help explain why better fuel atomization (and a wider spray pattern in a fuel injected engine, that covers more of the cylinder) will burn faster. there are more air and fuel molecules to mix and ignite, where in lean mixtures, there are less.

you will see that the ignition of the charge is focused more on one side, which takes longer to burn the entire mixture because it has to travel to the leaner side of the chamber and ignite the more widely spaced air and fuel particles. had the incoming charge been more uniform and been able to ignite in a wider pattern across the cylinder, it would burn faster. so, less air/fuel mixture to burn, the longer it takes to burn it.

thus, beginning the burn earlier in the process gives the cylinder more time to burn the lean mixture, leading to a more complete burn and better low rpm combustion, which i can personally attest to. to be very unscientific, with less timing at idle, my engine smells very rich, with more timing, the rich smell is gone. it's because the lean charge, which wasn't burning quickly enough at low rpm, is now being combusted more completely. idle mixture has some affect by adding or subtracting fuel to the incoming air, but it's a fine tuning that should be done after your timing is set. this is why everyone will tell you, changing the settings on your carb means nothing if your timing isn't right.